Electrical amplifier circuit



June s, 1928. 1,672,811

- L. M. HULL ELECTRICAL AMPLIFIER CIRCUIT Filed July 20. 1927 gwvemtoz:

- invention.

Patented June 5, 1928.

UNITED STATES PATENT OFFICE.

LEWIS M. HULL, OI BOONTON, NEW JERSEY, ASSIGNOR '10 RADIO FREQUENCY LLB- ORATORIES INCORPORATED. OF BOONTON, NEW JERSEY, A CORPORATION OF NEW JERSEY.

ELECTRICAL AMPLIFIER CIRCUIT.

Application filed July 20, 1927. Serial No. 207,288;

This invention relates to electrical amplifier circuits of the type employing a vacuum tube including an emitting cathode, an

anode, and one or more additional elements or grids. It relates especially to a balanced amplifier circuit of that type, and provides means for reducing or eliminating retroaction from the output circuit of the amplifier stage to the input circuit thereof. Such re troaction may be caused by coupling due to capacities between the elements of the vacuum tube and their associated connectlons, and especially by coupling due to the capacity between the anode and controlelement, or grid, of the vacuum tube.

Referring now to the drawings Fig. 1 shows a single amplifier stage embodylng my A three-electrode'vacuum tube of standard type is employed in this particular example. Fig. 2 shows a specific exam ple of the application of my invention to a four-electrode or double-grld type of electron tube. Fig. 3 shows a modification of the circuit of Fig. 1. Fig. 4 shows a cascaded arrangement embodying the circuit of Fig. 1.

The input circuit of the amplifier stage may assume any desired form. Referring to the arrangement shown in Fig. 1, the input circuit is connected between the control grid, G, and cathode, F, of the vacuum tube. The input circuit is here illustrated as a. tuned one, including coil L, and variable condenser C A plate coil, L is connected between anode and cathode, and any suitable source of space current, asa B-battery or socket power device may be included in circuit therewith. Coupled to the coil L with mutual inductance M is acoil L These coils have negative coupling, that is, one coil is wound or connected in reverse sense as regards the other. so that if the coils were connected in series the coupling between them. would produce voltages at their unconnected ends which would be approximately opposite in phase. While it is possible to employ substantially unity coupling between coils L, and L it is notnecessary to do so. and I prefer to employ substantially less than'unity coupling, and so avoid the high losses which are apt to take place in practice in the case of substantially unity coupling between reversed coils. In series with the coil L are connected the coil L and the tuning condenser C The coil L is preferably not coupled to either the coil L, or the coil L,, the coupling between the coil L and the coils L and L being in any case ne ligible. A balancing condenser may e connected between the grid Cr and a point, X, between coils L and T1,, and serves to improve the action of the circuit. The cathode, F, is connected to a point between coil L and tuning condenser C and since the cathode may be grounded, the tuning condenser C may also be ounded. This may be of advanta e in die case of multlstage amplifiers emp oying ganged condensers.

The approximate balance condition for this circuit may be expressed:

M" c. a

where C re resents the natural capacity beand anode P, asshown by the L 18 pk L 206 pk L 14. 5 ah Coupling coefiicient M,,=45% (about) O,= 19 a (about) These fi res are only one illustration of values WhlCh I have employed successfully.

Figure 2 shows my invention applied to a tube of the. double-grid type. The circuit is arranged similarly to that shown in Fig. 1, in general. The. second grid, G may be connected to the point X, as regards alternating current, and may also be given a constant direct current blas, as shown, it deslred, by means of the battery D or otherwise. If the battery D is employed, a stoppage condenser C, should also be employed, so as to prevent the potential of the battery D from being impressed between the cathode and control grid of the succeeding vacuum tube in a cascaded arrangement. The capacity between grids G and Gr may be employed for balancin the circuit, if the tube an coils are proper y proportioned. It may be advlsable', however, under some condiloo tionsl to connect the additional "capacity C, between the grids G and G in order to an ent the natural capacity.

igure 3 shows a modification of the circuit of Figure 1, which ma be useful in special cases. I have found t at the balance of the circuit shown in Figure 1 may not be suificiently independent of frequency in all cases, for example, if a very wide range of frequencies is to be received, or extremely accurate adjustments are intended. For such cases, the arran cment of Figure 3 tends to make the ba ance complete. The coil L, is connected in series w1th the coil L,, and is coupled to the coil L in series with the balancing capacity C L, should be relatively small, and L, somewhat lar er than L,, so that the mutual inductance will have about the same value as the selfinductance L,. The object of this arrangement is to introduce into the balancing voltage impressed upon the grid G through the condenser C a small uadrature component, to compensate for t e slight quarature com onent introduced into t e plate voltage by tlie flow of plate current through the 001i L An exact opposition in phase bet-ween the two opposing voltages may thus be obtained. It is obvious that this arrangement is equally applicable to a double-grid type of tube.

Figure 4 shows a cascaded arrangement employing the circuit of Fig. 1. The grid and cathode of the second vacuum tube may be connected directly across the tuning condenser C,, as shown, or other methods of cascadin the stages may be employed if preferre It is of course understood that common B-batteries or socket power devices may be used, separate batteries being here shown for simplicity.

It is obvious that many variations and modifications of the circuits herein described fall within the scope of my invention.

I claim:

1. An electrical amplifier stage comprising, in combination, a vacuum tube including an anode. a cathode. a control grid, and a second grid, an input circuit for said amplifier stage. a plate coil connected between said anode and cathode, a second coil coupled to said plate coil in the reverse sense, athird coil connected in series with said second coil, :1: connection between said cathode and said third coil, and a connection between said second grid and'a point inter-' mediate said second and third coils.

2. An electrical amplifier stage comprising, in combination, a vacuum tube including an anode. a cathode, a control grid, and a second grid, an input circuit for said amplifier stage, a plate coil connected between said anode and cathode, a second coil coupled 'to said plate coil in the reverse sense, a third coil connected in series with said second coil, a connection between said cath\ ode and said third coil, a balancing capacity connected between said control grid and second grid, and a connection between said second grid and a chat intermediate said second and third cm s.

3. An electrical amplifier stage comprising, in combination, a vacuum tube including at least an anode, a cathode, and a control grid; an input circuit for said stage connected to said control grid; an output couplingcoil connected between said anode and said cathode; a second coil coupled to said output coil in the reverse sense; a third coil connected in series with said second coilv but having negligible electromagnetic coupling therewith; a connection between said cathode and said. third coil; and a balancing capacity connected between said rid and a point between said second and third coils.

4. An electrical amplifier stage comprising in combimition, a vacuum tube including at least an anode, a cathode, and a control grid; an in ut circuit for said stage connected to sai control grid; an output couplin coil connected between said anode and said cathode; a tuned circuit including in series a second coil coupled to said outut coil, a tuning condenser, and a thirdmoil aving negligible electromagnetic coupling with said output coil; and a balancing condenser connected between said control rid and the junction of said second and t ird coils.

5. An electrical amplifier stage including a vacuum tube having at least an anode, a cathode, and a control grid; an input circuit associated with said control grid; a tuned output circuit including in series a tunin condenser having one terminal connected to said cathode, and two coils having negligible electromagnetic coupling with each other, one of said coils being coupled to acoil connected between the said anode and cathode; and a capacitive impedance connected between said control grid and one of said coils, at such a point that said stage may be balanced to reduce retroactive effects due to coupling between said output and input circuits.

6. An anti-regenerative electric amplifier stage including a vacuum tube having at least an anode, a cathode, and a control grid; and capacitive coupling between said anode and said control grid; a coil connected in the anode circuit of said vacuum tube; a second coil coupled to said first coil; a third coil constituting with said second coil serially connected elements of a resonant ouput circuit; and a balancing capacity connected between said control grid'and a point in said output circuit; the ratio of the capacity of said balancing condenser to said capacity between said anode and control grid being substantially equal to the ratio of the mutual inductance between said second coil and said firslt coil to the self-inductance of said third co1 i 7. An electrical amplifier stage including a vacuum tube having an anode, a cathode, and a control grid; a tuned output circuit including two inductance coils having negligible electromagnetic coupling with each other and connected in series between the terminals of atuning condenser, said tuning condenser having one terminal connected to said cathode; and abalancing condenser connected between said control grid and the junction of said inductance coils in said tuned output circuit.

8. A cascadeamplifier comprising a plurality of stages each of which includes a vacuum tube having at least an anode, a cathode, and a control grid; atuned out-put circuit for the first of said stages includingtwo substantially non-coupled inductances connected between the control grid and the cathode of the vacuum tube in the second of said stages and a tuning condenser connect.- ed between the control grid and cathode of the vacuum tube in the second-of said stages; and a balancing condenser connected between the control grid of the vacuum tube in the first of said stages and such a. point in said tuned output circuit that currents flowing to said control grid of said tube in said first stage through said balancing condenser oppose retroactive current-s flowing to said control grid through the grid-anode capacity of said tube.

9. A cascade electrical amplifier comprising a plurality of stages each of which includes having a vacuum tube having an.

anode, a cathode. and a control electrode; a tuned output circuit including two substantially non-coupled inductances serially connected between the control grid and the cathode of the vacuum tube in the second of said stages; and a balancing condenser connected between the control grid of the vacuum tube in the first of said stages and the junction point of said two serially connected inductances in said output circuit. i

10. An electrical circuit comprising, in combination, an electron tube including at least an anode, a cathode, and a control means; an input circuit associated with said control means; a coil associated with said anode and cathode; a second coil coupled to said first coil in the reverse sense: a. third coil having one terminal connected to said second coil but having negligible electromagnetic coupling with said second coil; a capacitive impedance between said control means and said second and third coils; and a connection between said cathode and a second terminal of said third coil; whereby said electrical circuit may be balanced to'rcdncc efi'ccts other than those due to desired forward repeater action of said electron tube.

11. An electrical circuit comprising, in.

combination, an electron tube including at least an anode, a cathode, and a control means; an input circuit associated wit-h said control means; a coil associated with said anode and cathode; a second coil con led to said first coil in the reverse sense; a third coil having one terminal connected to said second coil but having negligible electromag netic coupling with said second coil; 11 tuning condenser connected between the noncommonterminals of said second and third coils; abalancing circuit including capacitive impedance, said balancing circuit being connected between said control means and the junction of said second and third coils; and a connection between said cathode and the junction of said third coil with said tuning condenser; whereby said electrical circuit may be balanced to reduce effects other than those due to desired forward repeater action of said electron tube.

'12. An electrical circuit comprising, in combination, an electron tube including an anode, a cathode, a control grid, and a second grid; an input circuit associated with said cont-r01 grid; a coil associated with said I ns J magnetic coupling with said second coil; a

balancing capacity connected between said control grid and the junction of said second and third coils; av connection between said cathode and a second terminal of said third coil; and means for imparting a positivge hias to the second grid of said electron tu e.

13. An electrical circuit comprising, in combination, an electron tube including an anode, a cathode, a control grid, and a second grid; an input circuit associated with said control grid; a coil associated with said anpde and cathode; a second coil coupled to said first coil in the reverse'sense a third coi] having one terminal connected to said second coil but having negligible electromagnetic coupling with sald second coil; a tuning. condenser connected between the non-common terminal of said second and third coils; a balancing capacity connected between said control grid and the junction of said second and third coils; a connection between said cathode and the junction of said third coil and said tuning condenser; and means for imparting a positive bias to the second grid of said electron tube.

14. A multistage electrical amplifier circuit comprising, in combination, a plurality of electron tubes each including at least an anode, a cathode, and a control electrode;

ill

anode and cathode of said first tube; a second coil electromagnetically coupled to said first coil in thereverse sense and having one terminal connected to the control electrode of the second of said electron tubes; a third coil having one terminal connected to a second terminal of said second coil and a second terminal connected to the cathode of said second tube, said third coil having negligible electromagnetic coupling with said second coil; a balancing circuit including capacitive impedance, said balancing circuit being connected between said control electrode of said first tube and the LEWIS M. HULL. 

